1. Filed of the Invention
The present invention relates to photoelectric conversion film laminated color solid-state imaging apparatus that receives the incident light of one color among the three primary colors with photoelectric conversion films laminated on a semiconductor substrate and receives the incident light of the remaining two colors transmitted by the photoelectric conversion film with a photoelectric conversion device formed on the semiconductor substrate, and in particular to photoelectric conversion film laminated color solid-state imaging apparatus that provides good color balance between a color signal obtained from the light received with the photoelectric conversion film and that obtained from the light received with the photoelectric conversion device.
2. Description of the Related Art
Single board type color solid-state imaging apparatus represented by a CCD type or CMOS type image sensor arranges in a mosaic three or four types of color signal filters on an array of pixels to undergo photoelectric conversion. This outputs a color signal from each pixel corresponding to each color filter. Through signal processing of these color signals, a color image is generated.
In the color solid-state imaging apparatus where color filters are arranged in a mosaic, almost two-thirds of the incident light is absorbed by the color filters in case the filters are of the primary colors. This worsens light use efficiency and lowers sensitivity. Another problem is that only a single color signal is obtained for each pixel. Thus, the resulting resolution drops and false colors appear conspicuous.
In order to overcome such problems, imaging apparatus that laminates three layers of photoelectric conversion film on a semiconductor substrate on which a signal read circuit is formed have been studied and developed (for example, refer to JP-T-2002-502120 and JP-A-2002-83946). The imaging apparatus includes for example a pixel structure in which are laminated photoelectric conversion films generating a signal charge (electron, positive hole) from blue (B) light, green (G) light and red (R) light sequentially from an incidence plane. Further, for each pixel, there is provided a read part capable of independently reading a signal charge generated from light at each photoelectric conversion film.
In the case of the imaging apparatus including such a structure, almost all light undergoes photoelectric conversion and read out which means that the use efficiency of visible light is nearly 100 percent and that the color signals of three colors R, G, B are obtained for each pixel. This generates an image with high sensitivity and high resolution (with inconspicuous false colors).
Imaging apparatus described in the JP-T-2002-513145 provides a triple well (photodiode) for detecting an optical signal in a silicon substrate. Depending on the depth of the silicon substrate, signals of different spectrum response (peaked at the wavelength of B (blue), G (green) and R (red) from the surface) are obtained. This takes advantage of the fact that the distance of entry of the incident light into the silicon substrate depends on the wavelength. This imaging apparatus, same as those described in JP-T-2002-502120 and JP-A-2002-83946, obtains an image with high sensitivity and high resolution (with inconspicuous false colors).
The imaging apparatus, described in JP-T-2002-502120 and JP-A-2002-83946 must laminate three layers of photoelectric conversion film on a semiconductor substrate in order and form longitudinal wiring connecting a signal charge per R, G, B generated on each photoelectric conversion film to a signal read circuit formed on the semiconductor substrate. The problem is that this type of imaging apparatus is difficult to manufacture and the corresponding manufacturing yield is low, which results in a higher cost.
In the imaging apparatus described in JP-T-2002-513145, blue light is detected by a photodiode at the shallowest part, red light by a photodiode at the deepest part, and green light by a photodiode at the intermediate part. For example, an optical charge is generated from green light or red light by the photodiode at the shallowest part. Thus, separation of the spectrum response characteristic of R signal, G signal and B signal is not sufficient enough and the resulting color reproducibility is low. Moreover, it is necessary to add or subtract the output signal from each photodiode in order to obtain the true R signal, G signal and B signal. This addition or subtraction process degrades the S/N ratio of the image signal.
In order to solve the problems with the imaging apparatus described in JP-T-2002-502120, JP-A-2002-83946 and JP-T-2002-513145, there has been proposed imaging apparatus described in JP-A-2003-332551. The imaging apparatus is a hybrid type of the incident light described in JP-T-2002-502120 and JP-A-2002-83946 and that described in JP-T-2002-513145. The imaging apparatus includes a single layer of photoelectric conversion film sensitive to green (G) on a semiconductor substrate and receives the incident light of blue (B) and red (R) transmitted by the photoelectric conversion film on a photodiode formed in the semiconductor substrate, same as a related art image sensor.
Since this type of imaging apparatus uses only one layer of photoelectric conversion film, the manufacturing process is simplified thus avoiding an increase in cost and a decrease in the yield. Green light is absorbed by the photoelectric conversion film so that separation of the spectrum response characteristic of the photodiodes for blue light and red light in the semiconductor substrate is improved. This improves color reproducibility and the S/N ratio.
While hybrid type imaging apparatus described in JP-A-2003-332551 is advantageous in terms of reduction of manufacturing cost and improvement of color reproducibility and the S/N ratio, it is accompanied by the problem described below.
As a signal read circuit provide on the semiconductor substrate of the hybrid type imaging apparatus, a CCD type signal read circuit (including a signal transfer path and a transfer electrode) and a CMOS type signal read circuit (including a MOS transistor and signal wiring) are available. In this example, the CMOS type signal read circuit is described.
(1) The photoelectric conversion characteristic of a photoelectric conversion film greatly varies with the electric field in the film (or voltage across terminals of photoelectric conversion film). In case light of the same intensity is incident, it is desirable to accumulate signal charges at a constant rate. In reality, as the signal charges are accumulated in the film, the voltage across the terminals of the film drops thus decreasing the rate of the amount of charges accumulated in the film.
As a result, the proportional relationship between the light intensity and the output signal is lost and the linearity of the photoelectric conversion characteristic is degraded. On the other hand, a photodiode provided on a semiconductor substrate does not present such a problem. Thus, in the hybrid type imaging apparatus, it is difficult to strike a balance between the red light and blue light detected by the photodiode and the green light detected by the photoelectric conversion film, which results in a lower picture quality.
(2) The light transmitted by the photoelectric conversion film includes red (R) light and blue (B) light since green (G) light is absorbed. In case R light and B light are received by a double-layered photodiode formed in a semiconductor substrate, the absorption coefficient of the semiconductor with respect to B light is greater than that of semiconductor with respect to R light. Thus, a photodiode closer to the surface of the semiconductor substrate has a relatively higher sensitivity to B light while a photodiode at a deeper part has a relatively higher sensitivity to R light.
Because the light incident on each photodiode contains no G light, color separation between R and B is improved although color separation is not good enough as far as the difference of the light absorption coefficient of a semiconductor substrate is used. For example, red light incident on a photodiode for blue color generates an optical charge thus failing to obtain acceptable color reproducibility.